Laboratory experiments and seismology have created a clear picture of the major minerals

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Elastic behavior and pressure-induced structural evolution of synthetic boron-mullite ''Al 5 BO 9 '' (a = 5.678(2) A ˚, b = 15.015(4) A ˚and c = 7.700(3) A ˚, space group Cmc2 1 , Z = 4) were investigated up to 7.4 GPa by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. No phase transition or anomalous compressional behavior occurred within the investigated P range. Fitting the P-V data with a truncated second-order (in energy) Birch-Murnaghan Equation-of-State (BM-EoS), using the data weighted by the uncertainties in P and V, we obtained: V 0 = 656.4(3) A ˚3 and K T0 = 165(7) GPa (b V0 = 0.0061(3) GPa -1 ). The evolution of the Eulerian finite strain versus normalized stress (f E -F E plot) leads to an almost horizontal trend, showing that a truncated second-order BM-EoS is appropriate to describe the elastic behavior of ''Al 5 BO 9 '' within the investigated P range. The weighted linear regression through the data points gives: F E (0) = 159(11) GPa. Axial compressibility coefficients yielded: b a = 1.4(2) 9 10 -3 GPa -1 , b b = 3.4(4) 9 10 -3 GPa -1 , and b c = 1.7(3) 9 10 -3 GPa -1 (b a :b b :b c = 1:2.43:1.21). The highest compressibilities observed in this study within (100) can be ascribed to the presence of voids represented by five-membered rings of polyhedra: Al1-Al3-Al4-Al1-Al3, which allow accommodating the effect of pressure by polyhedral tilting. Polyhedral tilting around the voids also explains the higher compressibility along [010] than along [001]. The stiffer crystallographic direction observed here might be controlled by the infinite chains of edge-sharing octahedra running along [100], which act as ''pillars'', making the structure less compressible along the a-axis than along the b-and c-axis. Along [100], compression can only be accommodated by deformation of the edge-sharing octahedra (and/or by compression of the Al-O bond lengths), as no polyhedral tilting can occur. In addition, a comparative elastic analysis among the mullite-type materials is carried out.

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Elastic and structural behavior of a natural tetragonal leucite from the volcanic Lazium district (Italy) were investigated at high pressure by in situ single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions. A first-order phase transition, never reported in the literature, was observed at P = 2.4 ± 0.2 GPa from tetragonal (I4 1 /a) to triclinic symmetry (analysis of diffraction intensities suggests the space group P1), accompanied by a drastic increase in density of about 4.7%. The transition pressure was bracketed by several measurements in compression and decompression. No further phase-transition has been observed up to 7 GPa. Fitting a second-order Birch-Murnaghan equation of state (BM-EoS) to the pressure-volume data of the tetragonal polymorph, we obtain K 0 = 41.9(6) GPa and K′ = 4 (fixed). In the case of the triclinic polymorph, a second-order BM-EoS gives K 0 = 33.2(5) GPa. The eulerian finite strain (f e ) vs. normalized stress (F e ) curves were calculated for the low-and high-P polymorphs, providing F e (0) = 42 (1) and F e (0) = 33.2(4) GPa, respectively. The axial bulk modulus values of the tetragonal polymorph, calculated with a linearized BM-EoS, are K 0 (a) = 34.5(5) and K 0 (c) = 78(1) GPa. For the triclinic polymorph, we obtain K 0 (a) = 35.9(5), K 0 (b) = 34.9(7), and K 0 (c) = 35.5(7) GPa. The elastic behavior of the low-P polymorph appears to be more anisotropic than that of the high-P polymorph. The HP-crystal structure evolution of the tetragonal polymorph of leucite was studied on the basis of six structural refinements at different pressures between 0.0001 and 1.8 GPa. The main deformation mechanisms at high-pressure are due to tetrahedral tilting, giving rise to an increase of the ellipticity of the four-and six-membered rings of the tetrahedral framework. The T-O bond distances are practically invariant within the stability field of the tetragonal polymorph. The complex P-induced twinning, due to the tetragonal → triclinic phase-transition, and the low quality of the diffraction data at pressure above the phase-transition, did not allow the refinement of the crystal structure of the triclinic polymorph.

The high-pressure elastic behavior and the P-induced structure evolution of a natural cancrinite from Cameroun {Na 6.59 Ca 0.93 [Si 6 Al 6 O 24 ](CO 3 ) 1.04 F 0.41 ·2H 2 O, a = 12.5976(6) Å, c = 5 .1168(2) Å, space group: P6 3 } were investigated by in situ single-crystal X-ray diffraction under hydrostatic conditions up to 6.63(2) GPa with a diamond-anvil cell. The P-V data were fitted with an isothermal Birch-Murnaghan type equation of state (BM EoS) truncated to the third order. Weighted fit (by the uncertainty in P and V) gave the following elastic parameters: V 0 = 702.0(7) Å 3 , K V0 = 51(2) GPa, and K V = 2.9(4). A linearized BM EoS was used to fit the a-P and c-P data, giving the following refined parameters: a 0 = 12.593(5) Å, K a0 = 64(4) GPa, K á = 4.5(9), for the a-axis, and c 0 = 5.112(3) Å, K c0 = 36(1) GPa, K ć = 1.9(3) for the c-axis (elastic anisotropy: K a0 :K c0 = 1.78:1). A subtle change of the elastic behavior appears to occur at P > 4.62 GPa, and so the elastic behavior was also described on the basis of BM EOS valid between 0.0001-4.62 and 5.00-6.63 GPa, respectively. The high-pressure structure refinements allowed the description of the main deformation mechanisms responsible for the anisotropic compression of cancrinite on (0001) and along [0001]. A comparative analysis of the structure evolution in response of the applied pressure and temperature of isotypic materials with cancrinite-like topology is carried out.

The crystal-structure, crystal-chemistry, and low-temperature behavior of a natural phillipsite-Na from the "Newer Volcanic Suite," Richmond, Melbourne district, Victoria, Australia [K 0.75 (Na 0.88 Ca 0.57 ) Σ1.45 (Al 2.96 Ti 0.01 Si 5.07 ) Σ8.04 O 16 ·6.2H 2 O (Z = 2), a = 9.9238(6), b = 14.3145(5), c = 8.7416(5) Å, β = 124.920(9)°, and V = 1018.20(9) Å 3 , space group P2 1 /m], have been investigated by means of in situ single-crystal X-ray diffraction, thermogravimetric analysis, and electron microprobe analysis in the wavelength dispersive mode. Two accurate structural refinements have been obtained on the basis of single-crystal X-ray diffraction data collected at 298 and 100 K, with: R 1 (F) 298K = 0.035, 3678 unique reflections with F o > 4σ(F o ) and 195 parameters, and R 1 (F) 100K = 0.035, 3855 unique reflections, F o > 4σ(F o ) and 195 parameters. In both refinements, the residuals in the final difference Fourier maps are <1 e -/Å 3 . A configuration of the extra-framework population different from that reported in previous studies is found at room temperature, with two possible sites for potassium (K1 and K2), one sodium/calcium site (Ca), and seven independent sites partially occupied by water molecules (W1, W2, W3, W4, W4′, W5, and W6). The low-temperature refinement shows that the framework component of the phillipsite structure is maintained within the T-range investigated. However, a change in the configuration of the extra-framework content occurs at low temperature: the occupancy of site K2 drastically decreases, while that of site K1 increases, the Ca site is split into two sub-sites (Ca1 and Ca2) and the number of water molecule sites decreases to six (W1, W2, W3, W4, W5, and W6). The rearrangement of the extra-framework population at low temperature is likely due to the change in shape (and size) of the micropores by tetrahedral tilting. The evolution of the "free diameters" with temperature shows that an "inversion" of the ellipticity of the eight-membered ring channel along [010] occurs. The evolution of the unit-cell parameters with T (measured at 298, 250, 200, 150, and 100 K) shows a continuous and linear trend, without evident thermo-elastic anomalies. The axial and volume thermal expansion coefficients (α j = l j -1 ⋅∂l j /∂T, α V = V -1 ⋅∂V/∂T) between 100 and 298 K, calculated by weighted linear regression, yield the following values: α a = 1.8(1) × 10 -5 , α b = 1.2(1) × 10 -5 , α c = 1.1(1) × 10 -5 , and α V = 3.7(1) × 10 -5 K -1 . The thermal expansion of phillipsite is significantly anisotropic (α a :α b :α c = 1.64:1.09:1).

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